This invention relates generally to machines and procedures for incremental printing of text or graphics on printing media such as paper, transparency stock, or other glossy media; and most particularly to a scanning thermal-inkjet machine and method that construct images from individual ink spots created on a printing medium, in a two-dimensional pixel array. The invention is also believed applicable to sister technologies such as the hot wax transfer method. To optimize image quality the invention employs printmode techniques that are in some cases substantially randomized and in other cases arbitrary, but preferred embodiments of some facets of the invention invoke such printmode techniques through upstream tonal-level strategies.
A basic goal for these procedures and machines is very high quality in printed images, using a relatively inexpensive printer. Incremental printing nowadays is generally accomplished through digital manipulation of image data in one or another type of electronic digital microprocessor.
All such manipulation, including the stages discussed below under the conventional designations of xe2x80x9cimage processingxe2x80x9d and xe2x80x9cprintmaskingxe2x80x9d, can be performed in a host computer, e.g. in software that operates an attached printer, or can be built into the printer itselfxe2x80x94but most commonly is shared between the two. In still other systems some of the manipulation is performed in yet another distinct product, such as for example a so-called xe2x80x9craster image processorxe2x80x9d (RIP) to avoid overcommitting either the computer or the printer.
For operations performed within the printer or within an RIP, as is well known, the product may contain either a general-purpose digital processor running programs called xe2x80x9cfirmwarexe2x80x9d, or an application-specific integrated circuit (ASIC) manufactured to perform only specific functions of particular printers or RIPs. In some cases the printer or RIP may use both a firmware subsystem and an ASIC.
Image processingxe2x80x94The fundamental task of all these devices is receiving data representing a desired image and developing from those data specific moment-by-moment commands to a printing mechanism. This task, for purposes of the present document, will be called xe2x80x9cimage processingxe2x80x9d.
Such processing typically includes, at the outset, some form of darkness and contrast control or adjustment. In a color printer, this preprocessing stage analogously also includes color conversions and any needed color corrections. For purposes of generality in the remainder of this document (except where otherwise clear from the context), the terms xe2x80x9ccolorxe2x80x9d and xe2x80x9ccolorimetricxe2x80x9d will be used to encompass nonchromatic as well as chromatic colors, color specifications and the color manipulations just mentioned. Such preprocessing can handle both user-desired color modification and any known mismatch between an input-image color specification and the operating color space and gamut of the printer.
Next downstream from contrast, darkness and other color correctionsxe2x80x94and particularly important for images other than textxe2x80x94image processing also includes rendering or rendition techniques (such as dithering of error diffusion). A rendition stage may be regarded as having two principal functions, both directed to making spatial assignments of color ink spots to particular pixels.
First, it attempts to implement the relatively continuous or very fine tonal gradations of a photograph-like image, in terms of the relatively limited number of gradations which a typical inexpensive printer can produce. A digital file in a computer ordinarily is able to represent fine tonal gradations quite accurately, since data formatsxe2x80x94although digitalxe2x80x94usually allow for at least 256 distinct tonal levels between, for instance, pure white and dead black.
Second, in a color printer, rendition also attempts analogously to implement the relatively huge number of chromatic colors which a computer can invoke. Rendition must accomplish this in terms of the relatively limited number of chromatic colors which a typical inexpensive printer can produce.
Applying the broader definition of xe2x80x9ccolorxe2x80x9d suggested four paragraphs above, these two functions essentially collapse into a single broader functional concept. In other words, in the technical parlance of color science for incremental printing, both these functions may be regarded as implementing complex multilevel xe2x80x9ccolorxe2x80x9d values, in a printing system that can directly produce only a very limited number of xe2x80x9ccolorxe2x80x9d values.
Bandingxe2x80x94An obstacle to highest-quality printing is caused by repeating failure of particular elements of the print mechanisms to mark properlyxe2x80x94or to mark consistently with other elements. Periodic artifacts arise from constant or repeating errors of inkjet trajectory, pen positioning and speed, and printing-medium positioning and speed.
For instance malfunction or misalignment of a particular inking nozzle or the like can leave a generally consistent white or light pixel row across every image region where that particular element (e.g. nozzle) is supposed to mark. In the case of misalignment, the same problem also produces excess inking across some nearby region where the same element should not be marking.
This very simple example is only meant as a basic introduction to the concept of banding. As will shortly be seen, banding encompasses patterned artifacts that are far, far more complicated, bewildering and difficult to trace, to comprehend or accordingly to eliminate.
Image regions are not all equally affected by such defects. The visual impact or significance of banding problems, or more generally of dot-placement errors, varies with the tonal level or in other words dot density within an image.
We can define three regions of a tonal ramp, based on the amount of white space:
(1) highlights: These areas have ample white space and to the naked eye exhibit little in the way of banding or other dot-placement artifacts.
Such artifacts are of course present, but hard to seexe2x80x94because small differences in dot position can represent only a relatively small fractional change (or none) in the large amount of white space that is seen. Furthermore, because the dots that are present are so far apart, and usually irregularly located, they fail to form a visual frame of reference within which a person can detect placement errors directly.
(2) midtones: These parts of the tonal range are most sensitive to banding because they have small amounts of white space in conjunction with moderate amounts of dot-filled space.
Dot-placement errors are highly visible because small differences in dot position can have a large effect on how much white space is visiblexe2x80x94and in many situations also a disproportionate effect on the exact appearance of the moderate amount of dot-filled space. Coalescence contributes further to the conspicuousness of banding and graininess because dots clump together.
(3) saturated areas: These segments of the tonal range have almost no white space showing through, and again as in the highlights tend to exhibit minimal banding effects.
The large amount of colorant on the printing medium hides dot placement errorsxe2x80x94with the exception of print-medium advance problems. Interactions between the colorant and the printing medium, however, can lead to flood banding and coalescence.
As a practical matter, the boundaries of these tonal-range segments depend in part upon the nature of the image being printed, as well as the exact character of the dot-placement errors produced by a particular printhead. Therefore these regions of the tonal ramp can be defined neither sharply nor generally.
As a rule of thumb, however, for purposes of placement-error visibility the midtone region has very roughly more than one single printed dot per four pixelsxe2x80x94but, at the saturated end of the range, very roughly more than one single dot subtracted from full coverage, per four pixels. For example in a four-level (including zero) system, since the maximum number of dots in each pixel is three, the maximum inking in four pixels is 3xc3x974=12 and the upper limit of the midtone region is 12xe2x88x921=11 dots per four pixels.
In other words, the high-visibility range lies above approximately twenty-five percent coverage in single dots, but below approximately twenty-five percent in single dots deducted from the maximum possible inking level. Again, in practice the range defines itself in a functional way and not exactly in numerical terms.
Inking and coalescencexe2x80x94To achieve good tonal gradations and (for chromatic-color printers) vivid colors, and to substantially fill the white space between addressable pixel locations, ample quantities of colorant must be deposited. Doing so, however, generally requires subsequent removal of the water or other basexe2x80x94for instance by evaporation and, for some print media, absorptionxe2x80x94and this drying step can be unduly time consuming.
In addition, if a large amount of colorant is put down all at substantially the same time, within each section of an image, related adverse bulk-colorant effects arise. These include so-called xe2x80x9cbleedxe2x80x9d of one color into another (particularly noticeable at color boundaries that should be sharp), xe2x80x9ccocklexe2x80x9d or puckering of the printing medium, and even xe2x80x9cblockingxe2x80x9d or offset of colorant in one printed image onto the back of an adjacent sheet. In extreme cases such blocking can cause sticking of the two sheets together, or of one sheet to components of the printer apparatus.
All these conditions of coursexe2x80x94like the banding problem discussed in the preceding subsectionxe2x80x94defeat the objective of providing the highest practicable quality of printing in a relatively economical printer. Earlier efforts in this field have attempted to address these obstacles.
Printmodesxe2x80x94One useful known technique for dealing with both the above-described problems (i.e., banding and coalescence) is laying down in each pass of a printhead only a fraction of the total colorant required in each section of the image. Any areas left white or light in each pass tend to be filled in during one or more later passes.
These techniques, known as xe2x80x9cprintmodesxe2x80x9d, not only tend to control bleed, blocking and cockle by reducing the amount of colorant that is deposited on the page essentially all at once, but also help greatly to conceal banding effects. Preferably the several printing passes are overlapping, so that each swath of colorant tends to hide the kinds of banding due to periodic errors in printing-medium advance mechanisms.
For instance, even blank space between the edges of two inaccurately abutting swaths are usually covered by at least some colorant that is well within the boundaries of at least one other swath. Depending on the total number of passes, such blanks may be covered by as many as e.g. three other swathsxe2x80x94in a four-pass printmodexe2x80x94or even more. To put it another way, only one in four drops is missing along such a xe2x80x9cblankxe2x80x9d pixel row, and the nonuniformity is far less noticeable.
The specific partial-inking pattern employed in each pass, or in all of the passes considered in the aggregate, is called a xe2x80x9cprintmaskxe2x80x9d. (In some writings the term xe2x80x9cprintmaskxe2x80x9d is reserved for only the patterns used in each pass of a repeating set, while different terminology, xe2x80x9cshingle maskxe2x80x9d, is used to refer to the overall pattern of masks for all passes. This document follows the simpler usage of xe2x80x9cprintmaskxe2x80x9d for both.) The way in which these different patterns or masks add up to a single fully inked image is the xe2x80x9cprintmodexe2x80x9d.
Whereas the image-processing stage establishes spatial assignments of color spots to pixels, the printmasking stage establishes temporal assignments of color spots as among the several printing passes that have access to each pixel. Printmasking is ordinarily downstream from image processing.
Random maskingxe2x80x94Although printmode techniques are very powerful, it has been noticed that they fail to fully eliminate the effects of the previously described underlying periodic errors, and in some cases may even contribute to certain kinds of periodic artifacts. Early efforts to deal with these observations focused upon the regular, systematic character of the first generations of printmasks.
It was thought that the tendency of spurious printout patterns to persist or to be accentuated by printmasking was simply due to that regularity within the masks themselves. Because of this, considerable recent attention has been directed to randomization of the printmasking stage.
Some such efforts are reflected in the previously enumerated patent documents relating to randomized masks, randomized printmodes, and location rules. As those documents show, these efforts have been successful in eliminating systematic-appearing patterns within individual mask cells. Although such patterns within each cell have been replaced by irregular, random, wispy shapes, the shapes are ordinarily inoffensive when considered one cell at a time.
The improvement available through randomization, unfortunately, heretofore has been limited because printmasks are effective in hiding dot-placement errors only within the dimensions of the mask. In other words, the irregular, random shapes just mentioned become conspicuous and often unesthetic when repeated dozens of times across the width and down the height of an image.
Therefore a maximal improvement seemed to require that the printmask patterns be reasonably large in comparison with the overall imagexe2x80x94for instance, a pattern width equal to one-third or more than one-half of the total image width. More recent work (such as reported, for instance, in the copending applications of Garcia) suggests that the eye is insensitive to printmask size increase beyond roughly two and a half centimeters (one inch).
Even this size range, however, is on the order of hundreds of pixelsxe2x80x94and printers that can store and use large printmasks tend to be uncompetitively expensive. Most efforts have accordingly focused upon printmasks no wider than sixteen or thirty-two pixels.
Such widths are typically only a very small fraction of a full image width. Therefore such printmasks heretofore are necessarily replicated across the imagexe2x80x94with like considerations for the vertical dimensions leading to a similar replication down the image. The result, as mentioned above, is a repeating pattern (FIG. 1) that is all too easily seen in the midtones.
The illustration was made with an eight-by-eight pixel mask used to print an area fill that has one dot in each of four pixels of a superpixelxe2x80x94i.e., four dots total for each superpixel. This xe2x80x9clevel fourxe2x80x9d tone is well within the midtone range extending very roughly from two to twenty-seven single dots in each four pixels.
Artifacts such as those shown in FIG. 1 arise from interaction of repetitive mask usage with pen defects of directionality or inkdrop size. As this example demonstrates, development of new and better printmasks as such is likely to be only a partial solution to banding and other repeating artifacts in the midtones.
Superpixels or dither cells in image processingxe2x80x94In response to these seeming limitations of printmode strategy some very recent effort has focused upon the potentialities for randomization in the earlier, upstream, image-processing stage of the overall printing process. Pixel structures called xe2x80x9csuperpixelsxe2x80x9d and related structures called xe2x80x9cdither cellsxe2x80x9d, both previously used in the image-processing stages, have been explored as vehicles for introducing randomization to reduce banding.
The results, in terms of banding reductionxe2x80x94taught in the above-mentioned document Ser. No. 09/042,880 of Askeland and Doronxe2x80x94have been favorable. Some degradation of effective or perceived resolution, however, has been found to limit the acceptability of this technique for images that include sharply defined features.
Undercolor removal, or black replacementxe2x80x94One other area of innovations will be helpful as background to an understanding of the present invention. This subsection explains why undercolor and its so-called xe2x80x9cequivalentxe2x80x9d black are not equivalents colorimetrically, at least for purposes of the present document.
In regard to a chromatic-color image, the industrial term xe2x80x9cundercolorxe2x80x9d means the portion of each chromatic color (in the image) that is composed of equal parts of primary colors, e.g. red, green and blue. The magnitude of those equal parts, and therefore the magnitude of the undercolor, is equal to the magnitude of the smallest of the three primary constituents which make up each chromatic color.
It is well known that the magnitude of the undercolor isxe2x80x94in purest principlexe2x80x94equivalent to an equal amount of black. Therefore for instance in a printer an approximately equal hue appearance in an image can be obtained by replacing the undercolor with an equal amount of black.
For the sake of accuracy it is important to emphasize here that this theoretical equality is at best true in terms of hue appearance only. In various pragmatic ways the substitution of black for the tricolorant undercolor differs very importantly.
For example, in some circles it is held that the substitution of black ink for the supposedly equivalent aggregate of three colorants, in a real-world image-forming system, actually changesxe2x80x94and in fact degradesxe2x80x94the hue. This thinking is particularly applicable to midtone and heavy-shadow regions. This school of thoughtxe2x80x94which is creditable, and indeed rather sophisticated as to the more-serious artistic aspects of color reproductionxe2x80x94has it that the tricolorant form of the undercolor is in some way xe2x80x9cricherxe2x80x9d and therefore has greater esthetic or emotional impact.
Another difference is that in highlight regions the black substitution yields a much more granular appearance, due to the substitution of one dead-black dot for each three chromatically colored dots. As individual ink dots cannot be well resolved visually, their color is not perceptible. Thus scattered individual dots of the three chromatic constituent inks tend simply to appear gray, or in other words less dark than an individual dot of black ink.
Therefore the supposedly equivalent black dots, for the same average grayness, must be scattered more widely. It is this geometrical effect that accounts for the greater granularity. Particularly if the three individual colorants can be well spread about, in highlight regions, they are much better able to suitably render smooth gradations than the corresponding amount of black ink.
On the other hand, still other differences favor the use of black. The chemistry of black ink is different, typically simpler, than that of chromatic inks; thus drying times, interactions between inks, and interactions between ink and various different printing media, all tend to be much more controllable than for three separate chromatics. These factors all exert strong influences upon the overall color appearance of a printed imagexe2x80x94or, to use terminology most relevant to this document, upon the colorimetric character of the image.
Furthermore the volume of black ink is roughly one third the total volume of the three chromatic colorants; hence (even setting aside chemical differences) drying time is much shorter for black. For equal throughput, the black ink is therefore less susceptible to coalescencexe2x80x94which in various complicated ways can alter colorimetric properties drastically.
Based upon these several considerations it is well known to substitute black for undercolor where throughput without blocking or coalescence is of dominant importance. (Consideration of economy leads in the same direction, as fewer drops of black are required to obtain roughly equivalent gray-scale results, and even on a drop-for-drop basis black ink is often much less expensive.)
To the contrary it is known to refrain from such substitutionxe2x80x94and even to replace black already in an image with its three-ink undercolor xe2x80x9cequivalentxe2x80x9dxe2x80x94in highlight regions or elsewhere for best esthetic impact. From the discussion in this subsection it should be recognized, at the least, that black and its so-called xe2x80x9cequivalentxe2x80x9d undercolor are not xe2x80x9ccolorimetric equivalentsxe2x80x9d.
Conclusionxe2x80x94Repetitive patterns arising from systematic dot-placement errors, even in the presence of internally randomized printmask patterns, have continued to impede achievement of uniformly excellent inkjet printingxe2x80x94at high throughputxe2x80x94on all industrially important printing media. Thus important aspects of the technology used in the field of the invention remain amenable to useful refinement.
The present invention introduces such refinement. Before offering a relatively rigorous presentation, this section will preliminarily and informally introduce some of the thinking behind the invention. It is to be understood that this informal preamble is not a definition of the invention itself.
(1) Image fidelityxe2x80x94If the two or more such masks corresponded to significantly different tonal levels, or colorimetric levels, then this strategy would disrupt the imagexe2x80x94at the very least interfering with good resolution, as noted above for the superpixel method. The present strategy, however, instead provides a plurality of masks that are tonally or colorimetrically equivalent: different masks that would produce essentially the same color, given perfect pens, perfect pen-firing sequences and perfect media.
If in fact the pens and their firing operations were perfect, then there would be no visible pattern artifact to worry about in the first place, since (as mentioned above) such artifacts arise from interaction of repetitive mask usage with pen defects. The pens, firing and media are imperfect. As a matter of physical fact, then, the colorimetrically equivalent masks do not necessarily produce identically the same color, because they invoke different nozzles for production of a nominal color.
What the use of colorimetrically equivalent masks can accomplishxe2x80x94if properly usedxe2x80x94is to mix up, or scramble, the usage of the colorimetrically varying outputs produced by the different masks. When properly employed, this pixel-to-pixel mixing tends very strongly to break up the patterning. It does so with a degree of effectiveness that an automatic system can control to some extent by adjustment of the number of different masks used for each tonal level or colorimetric value.
(2) Pattern breakdownxe2x80x94Now, if a choice among colorimetrically equivalent masks were made systematically from pixel to pixel across rows and down columns, the result in many (but not all) systems would be merely a somewhat larger but still repeating pattern. To avoid such a pointless outcome, for many systems preferably the choice for each pixel is made in a substantially independent way, relative to the choices already made for previous pixels or those about to be made for later-processed pixels.
Perhaps ideally the choice for each pixel respectively is made randomly. In practice, however, a truly random choice may be relatively costly, and such masking also might compromise other key objectives of quality printing. The quality of the resulting images is equal or closely comparable if the choice is xe2x80x9crandomizedxe2x80x9d (as will be discussed below) or even is xe2x80x9csubstantially randomizedxe2x80x9d.
Now with these preliminary thoughts in mind, this discussion will turn to a more-formal presentation.
In its preferred embodiments, the present invention has several aspects or facets that can be used independently, although they are preferably employed together to optimize their benefits.
In preferred embodiments of a first of its facets or aspects, the invention is apparatus for printing desired images on a printing medium. The apparatus does so by construction of the images from individual marks formed in pixel arrays.
The apparatus includes some means for establishing plural selectable colorimetrically equivalent printmasks. For purposes of generality and breadth in discussing the invention, these means will be called the xe2x80x9cprintmask-establishing meansxe2x80x9d or more simply the xe2x80x9cestablishing meansxe2x80x9d.
The apparatus also includes some means for selecting printmasks, from among the plural selectable printmasks, for use in printing. Again for breadth and generality these means will be called the xe2x80x9cprintmask-selecting meansxe2x80x9d or simply the xe2x80x9cselecting meansxe2x80x9d.
By xe2x80x9ccolorimetrically equivalentxe2x80x9d is not meant xe2x80x9ccolorimetrically identicalxe2x80x9d, a more stringent criterion. Colorimetrically equivalent masks, for instance, may differ to some extent in their ability to suppress coalescencexe2x80x94particularly on the special media that are particularly favored for printing pictures that look like photographs. (On the other hand, colorimetrically identical masks are colorimetrically equivalent.) As pointed out earlier, xe2x80x9ccolorimetrically equivalentxe2x80x9d also does not encompass undercolor removal or black-to-undercolor replacement.
This document, including the appended claims, also uses the phrase xe2x80x9csubstantially colorimetrically equivalentxe2x80x9dxe2x80x94a slightly looser criterion. For purposes of the present document, it is defined as specifically encompassing a system designer""s discretion to deliberately incorporate some quite small variations. Such variations may include modifications in the numbers and locations of the inkdrops fired, as well as their temporal distribution.
Such deliberate variations may arise in two different ways. One may be a competitor""s desire to escape from the literal meaning of xe2x80x9ccolorimetrically equivalentxe2x80x9d to avoid the sweep of the appended claims.
The second, and perhaps a more salutary, object may be to superimpose an additional layer of variations upon those achieved by selection among colorimetrically equivalent masks. For instance such variations may be very helpful in avoiding tonal-step quantization artifacts, as will be discussed below in connection with error-diffusion embodiments of the invention.
The foregoing may constitute a description or definition of the first facet of the invention in its broadest or most general form. Even in this general form, however, it can be seen that this aspect of the invention significantly mitigates the difficulties left unresolved in the art.
In particular, this aspect of our invention introduces an entirely new way to suppress and hide banding and other repetitive artifacts. It does so by . . .
Although this aspect of the invention in its broad form thus represents a significant advance in the art, it is preferably practiced in conjunction with certain other features or characteristics that further enhance enjoyment of overall benefits.
For example, it is preferred that the apparatus also include a nonvolatile memory holding program instructions for automatic operation of both the printmask-establishing and -selecting means. It is also preferred that the apparatus include a printing stage for applying the selected printmasks in printing, to control forming of marks on such medium.
Preferably in the latter situation the apparatus also has a printer case, a pen carriage mounted for reciprocating motion in the printer case, and an advance mechanism in the printer case for effecting relative motion of the medium with respect to the pen carriage, along a direction of motion substantially orthogonal to the carriage reciprocating motion. In this case the apparatus additionally includes at least one pen carried on the carriage, in multiple passes across each pixel, for ejecting ink to form the marks on the medium. The printmask-establishing and -selecting means, and the printing stage, considered together, include at least one associated digital processor for controlling and coordinating the carriage, the advance mechanism and the pen or pens.
Still further as to the preference just described, the apparatus also preferably includes an image-processing stage that assigns inking spatially as among pixels; in this instance it is also preferable that each of the established printmasks sets temporal assignments, as among printing passes, of the spatially assigned inking. Another related preference is that the applying means include some means for employing the selected printmasks for successive pixels in a substantially randomized sequence.
The term xe2x80x9crandomizexe2x80x9d as used in this document is not to be misunderstood as limiting the invention to equipment or a method that is truly random. Such equipment is, on the other hand, within the scope of the word xe2x80x9crandomizexe2x80x9d.
Thus the term xe2x80x9crandomizexe2x80x9d as will be seen is intended to convey apparatus and method operating on the basis of a sequence that has no sensible or logical-appearing pattern. (Perhaps the ultimate test is a somewhat circular onexe2x80x94namely, whether there is any perceptible or significant pattern artifact in a resulting printed image.)
This definition in turn naturally encompasses sequences having at least major random contributionsxe2x80x94but also possibly satisfying certain constraints that disrupt the degree of perfection of the randomness. Another way to say the same thing may be that the apparatus and method are xe2x80x9cpseudorandomxe2x80x9d.
As suggested earlier in this subsection, a systematic selection among colorimetric equivalents, though undesirable in many systems, is acceptable in some systems. In preferred embodiments of some aspects of the invention, a somewhat systematic method of choice can be made to interact with characteristics of the image, to suppress pattern artifacts. For instance, one aspect of the invention for which this is true is error diffusion.
This point is further developed below. As will be seen, in some such cases a degree of randomization may arise in the process.
Still another preference, as to the first main facet or aspect of the invention, is that the establishing means establish printmasks that each occupy a very small fraction of the image width. It is still more highly preferable that the masks each occupy significantly less than six millimeters (one-quarter inch) in both width and height respectively.
Another preference is that the selecting means select the printmasks for successive pixels, from among the established printmasks, by a substantially randomized process.
Yet another preference is that the selecting means define an input image as an array of input colorimetric levels for printingxe2x80x94and selectably map each input colorimetric level to any one of a plurality of colorimetrically equivalent printmasks. In this situation it is even more highly preferable that the apparatus also include some means for establishing a plurality of colorimetrically equivalent tonal levels.
The concept of colorimetric equivalence, as used in this document, has been introduced above. Its application here to tonal levels (as distinct from printmasks) is straightforward.
In this case the selecting means also include some means for assigning each input colorimetric level of the defined input image independently to a particular one of the plurality of colorimetrically equivalent tonal levels. This assigning is done by a substantially randomized procedure.
As a result the selecting means and mapping means cooperate to automatically perform a substantially randomized assignment of each input level of the defined input image to a respective one of the plurality of colorimetrically equivalent printmasks. Thus this preference in essence, among other possible uses, provides a stratagem for using plural colorimetrically equivalent printmasks, and selecting them in a randomized way, but without obtaining direct access to the printmasking stage.
Such direct access is otherwise generally necessary, if one sets out to define equivalent masks and establish a methodology for selecting among them. Therefore this preferred approach is especially useful in situations that preclude such direct access, as for instance when it is desired to incorporate the present invention into a commercial product whose printmasking stage has been frozen with respect to additional engineering changes.
Yet another preference, as to first aspect of the invention, is that the printmask-defining means include some means for preparing the image using a pixel grid that is coarser than the available printer resolutionxe2x80x94and also means for constructing the plural colorimetrically equivalent printmasks by varying allocation of printer passes as among pixels of the coarser grid.
As will be seen, colorimetrically equivalentxe2x80x94or substantially equivalentxe2x80x94masks may be provided that:
(1) subdivide incoming source-image pixels into a finer pixel grid at the printer resolution, and parcel out the source-image pixel data differently into the finer printer-pixel grid (this is the preference described in the preceding paragraph); or
(2) perform essentially the same processing but simply in a different temporal sequence, or
(3) both subdivide incoming pixels and create different temporal sequences.
Still other methodologies for creating masks that are colorimetrically equivalent, or substantially so, are within the scope of certain of the appended claims.
In preferred forms of a second of its facets or aspects, our invention is a method for printing desired images on a printing medium, by construction from individual marks formed in pixel arrays. The method includes the step of establishing plural selectable colorimetrically equivalent tonal levels.
In addition the procedure includes the step of selecting tonal levels, from among the plural selectable colorimetrically equivalent tonal levels, for use in printing.
The foregoing may constitute a description or definition of the second facet of the invention in its broadest or most general form. Even in this general form, however, it can be seen that this aspect of the invention, too, significantly mitigates the difficulties left unresolved in the art.
In particular, as mentioned earlier, this establishment of plural colorimetrically equivalent levels offers a way of gaining access to a printmasking stage, indirectlyxe2x80x94for example to introduce selections as among plural masksxe2x80x94when it is not possible to gain such access directly. Other beneficial applications of this second main aspect of the invention may occur to those skilled in the art.
Although this second aspect of the invention in its broad form thus represents a significant advance in the art, it is preferably practiced in conjunction with certain other features or characteristics that further enhance enjoyment of overall benefits.
For example, it is preferred that the method also include the step of applying the selected levels in printing, to control forming of marks on the medium. In this latter case it is also preferred that the applying step include operating a printing mechanism such as described earlierxe2x80x94i.e., a pen carriage, a pen or pens, an advance mechanism and a digital processor or processors.
Another preference is that the applying step include employing the selected tonal levels for successive pixels in a substantially randomized sequence. Yet another preference is that the tonal-level selecting step assign inking spatially as among pixels; and that the method also include a printmasking step, following the selecting step, that sets temporal assignments, as among printing passes, of the spatially assigned inking.
Still other preference, as to the second main aspect of the invention, is that the printmasking step include establishing printmasks that each occupy a very small fraction of the image width. Another preference is that the selecting step include selecting tonal levels for successive pixels, from among the plural selectable colorimetrically equivalent levels, by a substantially randomized procedure.
An additional preference is that the selecting step include the substep of defining an image as an array of colorimetric levels for printing. Going along with this preference is another substep, namely mapping the plural selectable colorimetrically equivalent tonal levels to a plurality of colorimetrically equivalent printmasks, respectively.
In preferred embodiments of a third of its basic aspects or facets, the invention is apparatus for printing desired images on a printing medium, by construction from individual marks formed in pixel arrays. The apparatus includes a halftoning stage for establishing a respective tonal level for printing at each pixel in such array.
In addition the apparatus includes some means for establishing plural distinct selectable colorimetrically equivalent levels for use in the halftoning stage. For reasons outlined earlier these means will be called the xe2x80x9clevel-establishing meansxe2x80x9d or just the xe2x80x9cestablishing meansxe2x80x9d.
The apparatus also includes a printmasking stage for employing printmasks to establish temporal assignments of inking as among printing passes. The apparatus also includes some means for selecting tonal levels, from among the plural selectable colorimetrically equivalent levels, for use in the halftoning stage.
The foregoing may represent a definition or description of the third aspect of our invention in its most general or broad form. Even as so couched, it can be seen that the invention in this form importantly advances the art.
In particular, as before the establishment of plural distinct but colorimetrically equivalent levels provides a system designer with a handle that reaches into the later masking stage and can be used to manipulate masking details or other functions of that stage. Nevertheless we prefer to practice the invention with certain other features or characteristics that optimize the benefits of the invention.
In particular one preference is that the halftoning stage be an error-diffusion stage. In this case preferably that stage is a table-based error diffusion system.
In such a system, the level-establishing means include a lookup table that defines, for each input tonal level, a base level and an error value. The establishing means also include some means for defining plural colorimetrically equivalent base levels.
This preference is advantageous in that, besides reaching from the earlier halftoning stage into the later printmasking stage, the system does so by taking advantage of the flexibility usually available through access to the error-diffusion lookup table. Lookup tables are typically reserved out of software and electronics, even when software and electronics are closed to engineering changes. A table is ordinarily kept accessible for the very reason that many reasons for modifying data tabulations tend to arise long after first versions of products are released to market.
In the error-diffusion system under discussion, the table provides access to definition of equivalent printmasks for outputting the image to a printer mechanism. Meanwhile, as before, the upstream plural levels provide a means of defining equivalence of the masks, in a way that cannot be done directly when the error-diffusion algorithm is locked in, for example, an ASIC. Thus part of the strategy operates on the upstream level-establishing stage and the balance operates on and through the lookup table.
Another preference, still with respect to an error-diffusion stage, is that the selecting means assign different colorimetrically equivalent base levels to multiple successive colorimetrically adjacent input tonal levels. In this case it is preferable that the apparatus also include means for mapping each plural selectable equivalent level to a respective likewise-equivalent printmask. Another preference is that the selecting means be substantially randomized.
All of the foregoing operational principles and advantages of the present invention will be more fully appreciated upon consideration of the following detailed description, with reference to the appended drawings, of which: